1. Field of the Invention
This invention pertains generally to above ground fuel tanks for generators, and more particularly to a lightweight, low profile base tank with fire resistive, impact resistive and leak protection elements.
2. Description of Related Art
Many auxiliary generators that provide remote or backup power are mounted on a base tank as an integrated system. The base tank is required to provide sufficient fuel to the generator system to run for extended periods of time without refueling. Space for installation and access for auxiliary generators is often constrained by site features and facility design, limiting the equipment that may be used for positioning. For a given space, the dimensions and profile of the tank, along with the necessary mounting system for the generator, define the volume of fuel that may be accommodated in a base tank.
In most applications, aboveground tanks must have secondary containment to prevent fuel leaks to the environment and employ a double wall tank design. Underwriters Laboratory Inc. (UL) 142 standard (incorporated herein by reference) for steel aboveground tanks for flammable and combustible liquids is a safety standard that has been followed to construct commercially available double wall tanks for more than 14 years. A more recent UL 2085 standard for protected aboveground tanks (incorporated herein by reference) specifies limits to the heat transferred to the primary, or inner fuel tank, when exposed to a two-hour hydrocarbon pool fire. This standard further specifies a protection requirement from physical damage including projectile damage.
Commercially available base tanks constructed to the UL 2085 standard typically use a double wall metal tank with concrete, solidified foam or other solid insulating material in an interstitial space of about six inches to resist the heat of a two hour fire and provide damage and projectile protection. The fire resistant insulation is typically installed before the tank is transported to the site. Increased tank weight increases cost and complexity of installation, however. The relatively large interstitial space required for solid insulating material significantly increases the footprint of the base tank and decreases fuel volume for a given installation space. Furthermore, once installed, solid insulating material cannot be easily removed from the interstitial space for inspection or repair.
By way of example, U.S. Pat. Nos. 6,422,413 and 5,271,493 to Hall et al., incorporated herein by reference, teach using about six inches of poured concrete as an insulator. The concrete works well as a fire shield, however, the concrete also makes the tank extremely heavy and cumbersome to transport and install. Further, once the concrete has hardened, it cannot be readily removed for tank inspection or repair.
U.S. Pat. Nos. 6,026,975, 6,257,437 and 6,349,873 to Slater, incorporated herein by reference, disclose Perlite, Vermiculite, fire retardant polymeric foam, ceramic or cementitious materials such as regular concrete, sand or a cementitious material containing a aggregate as an insulator material in the interstitial space. Slater describes a solid insulation, preferably concrete, in an interstitial space, preferably 6 inches to provide the required fire resistance and impact resistance for the UL 2085 standard. Slater does not teach a tank embodiment with an interstitial space of less than 6 inches that meets the UL 2085 standard nor the use of a non-solid fire resistant insulation in the interstitial space.
Other methods have been employed to provide fire resistance to above-ground fuel tanks. For example, U.S. Pat. Nos. 5,285,920; 5,012,949; 5,038,456 and 4,989,750 to McGarvey et al., incorporated herein by reference, describe above-ground fire resistant tanks that have a sprayed-on exterior fire resistant intumescent material such as Chartek. McGarvey further teaches a double wall tank embodiment with a solid insulation material such as Vermiculite, foamed concrete, Fendolite, Styrofoam or pumice in the double wall space. McGarvey does not, however, teach a combination of exterior fire resistant material, interstitial insulation and support structure necessary to function as a generator base tank. Furthermore, McGarvey does not teach a non-solid insulation material for the interstitial space.
Although several above-referenced patents suggest materials other than A-36 mild steel for tank walls, such as plastic, fiberglass, or corrosion resistant steel, they do not suggest any particular type of steel that would provide the combined advantage of improved heat conduction, impact resistance, and as corrosion resistance to a water base fire resistant solution in the interstitial space. In fact, the corrosion resistant steels described do not exhibit those properties.
Furthermore, in order to support a generator on the top of a base tank, there must be a support structure extending from the equipment pad, to the mounts of the generator on the tank. For example, U.S. Pat. Nos. 6,026,975, 6,257,437 and 6,349,873 to Slater, incorporated herein by reference, disclose stiffening members for the top and bottom walls of the inner tank, top and side walls of the outer tank, and support beams along the top outer tank wall. These stiffening members provide support for the generator, however this external support configuration adds significant weight and size to the tank system and can interfere with generator maintenance access.
Therefore, there is a need for a lightweight low-profile base tank designed to meet the UL 2085 standard and support commercially available generators. Further, the capability to install a fire resistant material in the interstitial space after tank installation and inspect and repair the tank without dismantling is highly beneficial to overcome shortcomings of bulkier and heavier generator base tanks.